I can't sign in to the DART API

One of the following may be happening:

You're attempting to sign in to the DART API with your Google Account sign-in information. Unless you enter dfpapi.doubleclick.net in a browser, you need to use your legacy DoubleClick for Advertisers (DFA) or DART for Publishers (DFP) sign-in to access the API  even after you update your DFA/DFP sign-in to a Google Account. In other words, you'll use your Google Account to sign in to DFA/DFP, and your legacy DFA/DFP sign-in to access the API.

If you have updated your sign-in to a Google Account (because you use both the DART API and DFP UI)  you can sign in to dfp.doubleclick.net, select the DFP API sign-in (if you have more than one DFP sign-in associated to the same Google Account), and change the password in the My Profile section. You'll then be able to use your legacy DoubleClick username with the new password to access the DART API programmatically.

If you're an advanced user who can write API programs  you can use the following function of the OrganizationFactory class. Please see the API documentation for details.